Adam Hayat appeared in court on Wednesday to change his plea on three charges of possessing unregistered firearms.
Hayat was arrested at a hotel near Los Angeles International Airport on Feb. 16, 2017, a day after the explosives were found in Denver.
The 36-year-old is scheduled to be sentenced on August 14.
According to court records, police found the pipe bombs inside a closed ammunition case placed in the safe. They also found several metal pipes and empty rifle shell casings inside the hotel room.
A spokeswoman for the Office of the Federal Public Defender, which is representing Hayat, was not immediately available Thursday for comment.